Immune responses can be categorized according to patterns of cytokine production. Type 1 cytokines promote cell-mediated immunity, while type 2 cytokines mediate allergic responses. Children infected with respiratory syncytial virus (RSV) who develop acute bronchiolitis, rather than mild upper respiratory infection symptoms, have impaired type 1 immunity or augmented type 2 immunity.[7]
In addition to humoral responses, cell-mediated immunity appears to be important for recovery from certain respiratory viral infections. Impaired type 1 response may explain why immunocompromised patients have more severe viral pneumonias.
Respiratory viruses damage the respiratory tract and stimulate the host to release multiple humoral factors, including histamine, leukotriene C4, and virus-specific immunoglobulin E in RSV infection and bradykinin, interleukin 1, interleukin 6, and interleukin 8 in rhinovirus infections. RSV infections can also alter bacterial colonization patterns, increase bacterial adherence to respiratory epithelium, reduce mucociliary clearance, and alter bacterial phagocytosis by host cells.